Method for producing a shaped body from a polycondensate

ABSTRACT

The invention relates to a method for producing a shaped body, particularly a profiled piece, from a polycondensate, particularly a polyester such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT) or one of the copolymers thereof. According to the invention, the polycondensate is melted over the course of the method and, later, is rendered solid once again. The invention is characterized in that the total time, during which the temperature of the polycondensate over the course of the method is higher than the melting temperature of the polycondensate, is less than approximately 60 seconds. The method is preferably carried out by means of a multiple screw extruder, whereby the degassing and/or drying of the polycondensate ensues in the solid state at a pressure less than the atmospheric pressure and/or while adding an inert gas. The period of time during which the polycondensate dwells in the extruder after having been melted therein is less than approximately 15 seconds.

The invention relates to a method for manufacturing a molded part, in particular a profile, out of a polycondensate, in particular polyester, e.g., polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT) or one of its copolymers, wherein the polycondensate is melted and later solidified again during the process.

Such methods, in particular involving the use of PET, are known. The PET starting material is here derived directly from PET synthesis, or use is made of a recyclate, in particular a PET bottle recyclate (RPET), wherein work proceeds with the chips or flocks (flakes) obtained after washing and comminuting. In addition to any possible other undesired contaminants, these chips always contain slight quantities of water, wherein this water can involve residual water not yet expelled from the bottle washing process and/or water that newly penetrated the chips during their storage and transport. If these water-containing chips comprised of RPET or new PET are melted in the extruder to manufacture a molded body, the condensation reaction proceeds mainly in the reverse direction (equilibrium sets in), and hydrolysis predominates, so that the average chain length, i.e., the degree of polymerization, decreases, reducing the viscosity (indicated as the viscosity number or intrinsic viscosity (IV)) of the polymer. The more water and time available for the degradation, the more pronounced this hydrolytic degradation.

Previous efforts in prior art are geared toward reducing water content. The usual preliminary drying process is here aimed at achieving low values for water content down to under 100 ppm (typically about 30 ppm) to prevent too great an IV reduction during the subsequent melting of the PET and/or RPET at the usual retention times in the melted state, in which the reaction rates of the polycondensate are notable. Acceptable IV reductions measure about 0.05, from roughly 0.80 to 0.75. This extensive preliminary drying requires relatively high amounts of time and energy, and slows the process described at the outset if continuous operation with integrated preliminary drying is the objective.

Also known in prior art is to expose melted PET and/or RPET to a melt vacuum measuring a few mbar, or even under 1 mbar, for a prolonged period of time to remove the water released in the polycondensation reaction from the melt, thereby increasing the degree of polymerization. However, such a method is unsuitable for polyesters with a high viscosity, and in particular for RPET comprised of washed bottle chips as well, since the polycondensation reaction is always simultaneously accompanied by degradation reactions, leading to a deterioration in product properties, e.g., yellow value or acetaldehyde content.

DE10054226 describes a method in which hydrolytic degradation is minimized via the suitable selection of process parameters, in particular the retention time, and optionally a limited preliminary drying. However, this method only describes the manufacture of granulates, or of strands that are subsequently cut into granulates. Therefore, the description centers on the manufacture of an intermediate product that must be melted once again to generate a product, wherein the shape of the product is untreated to the shape of the granulate. The disadvantage to this is that the polycondensate as a whole must be melted at least twice, thereby extending the overall period for which the polycondensate is kept in the melt, which results in a higher level of degradation as a whole during the manufacture of the end product.

By contrast, the object of the invention is to design the method described in prior art in such a way that no greater IV reduction than in prior art arises given only a partial preliminary drying, further degradation reactions resulting in a deterioration in product properties are minimized, and the method enables the direct manufacture of molded parts from which the finished product can be manufactured without repeated melting.

The advantage to this is that a product can be obtained from a polycondensate starting material given a minimal overall retention time of the polycondensate in the melt, thereby minimizing both hydrolytic and thermooxidative degradation. For example, this results in improved color values and, during polyethylene terephthalate processing, lower acetaldehyde values.

Finished products or semi-finished products that can be converted into finished products in processing steps like cutting, bending or thermoforming, are referred to as molded parts or profiles. The shape of the end product here corresponds to the shape of the molded part, or is directly derived from the shape of the molded part.

For example, a frame comprised of a complex profile rod can be manufactured by cutting and joining several parts together. In like manner, a round profile can be used to manufacture round panes or plates by cutting the profile rod essentially transverse to the manufacturing direction. Another example involves the manufacture of hollow items, such as shells or beakers, from a film or plate by pressing or pulling the film or plate into or over a molding tool. It may here be necessary to heat the polycondensate molded part to the softening point. However, heating in excess of the polycondensate melting point does not take place. The hollow items can again be finished products or semi-finished products, e.g., which are again subjected to dimensional adjustment via stretch blow molding.

This object is achieved by the method according to claim 1, specifically by limiting the time for which the polycondensate to be processed is present in the melt at or over the melting point to less than 60 seconds. As a result, the polycondensate no longer has sufficient time to react in a predominately hydrolytic manner owing to its relatively high water content. This makes it possible to partially eliminate the time-consuming and energy-intensive preliminary drying step, and tangibly less hydrolysis takes place while processing in the melted state, despite the relatively high share of water in the starting polycondensate. For example, the IV degradation can be kept to a small, acceptable value for PET or RPET, even given a relatively high water content of roughly 600 ppm.

The overall time for which the temperature of the polycondensate exceeds the melting point of the polycondensate during the process is preferably kept under about 30 seconds in the method according to the invention. One can here use a residual water content in excess of 200 ppm (w/w) in the melt without having to accept a IV reduction of greater than 0.05.

Polyethylene terephthalate (PET) is a preferred polycondensate for many applications.

In its initial form, the polycondensate can be present as a loose material having an apparent density ranging from 200 kg/m³ to 950 kg/m³, in particular in the form of granulate, powder, flocks or chips, wherein the latter typically consist of bottle recyclate (RPET).

The polycondensate starting material is preferably subjected to partial preliminary drying before melting. In this way, combining a relatively uncomplicated partial drying process and the brief retention time in a melted state makes it possible to obtain an end product with a low IV degradation.

In a special embodiment, the method includes a degassing step for removing volatile contaminants and/or decomposition products from the polycondensate.

The polycondensate is preferably melted using a two-screw or multi-screw extruder, in particular a ring extruder. In the ring extruder, the ratio between the surfaces that actively impinge on the product to be processed and the volume of the product to be processed is especially large, thereby yielding a higher degassing capacity and narrower retention time spectrum at a prescribed overall, and finally a shorter retention time as a whole than for conventional two-screw extruders.

The polycondensate is preferably introduced into the extruder in a solid state, and the polycondensate is heated to a temperature below the melting point, during which the polycondensate is degassed and/or dried. The polycondensate is here degassed and/or dried in a solid state at a pressure of several mbar (typically 20-300 mbar) below atmospheric pressure and/or with the addition of an inert gas.

In particular, the method according to the invention is characterized by the fact that the overall time for which the polycondensate is present as a melt during the process consists of a first time segment in which the polycondensate still remains in the extruder after melted in the extruder, and a second time segment for which the still melted polycondensate is processed outside the extruder, wherein the first time segment preferably measures less than about 15 seconds. A retention time of the melt in the extruder of less than about 10 seconds is particularly advantageous.

When processing an RPET or an even slightly preliminarily dried PET, it may be advantageous to degas the melt after melting in the extruder in order to remove potential contaminants or even portions of dissolved water from the melt. A vacuum of several 10 mbar absolute pressure (typically 30-130 mbar) is sufficient for this purpose. A further reduction in pressure is often not necessary, and hence also not desired for process cost considerations. Nonetheless, a vacuum of only a few mbar or even under 1 mbar absolute pressure can be used to reduce the partial pressure of the contaminants to be removed over a short time.

The method according to the invention is also suitable for incorporating additives.

The additives can be incorporated before melting, either together with the polycondensate, or via a separate metering and loading device.

In this case, the additives are optimally mixed simultaneously by the kneading elements 13 during the melting process. The additives can also be incorporated after melting in the extruder. For example, the additives are incorporated by means of a lateral loading device. Additional kneading or mixing elements can optionally be provided in the extruder, so that the additives are optimally mixed. In special cases, the additives can also be added only after the extruder.

Suitable additives include dyes and pigments, UV blockers, processing aids, stabilizers, impact modifiers, chemical and physical foaming agents, fillers like nucleating agents, particles that improve barrier or mechanical properties, reinforcing bodies, such as balls or fibers, along with reactive substances, for example oxygen absorbers, acetaldehyde absorbers or molecular weight-increasing substances, etc.

The additives can be incorporated alone or as part of an additive packet. Several additives are used to fabricate the additive packet. In addition, use can be made of a carrier material that allows incorporation of all additives. The additive packet can be present both as a homogenous powder or granulate, or as a simple additive mixture.

Processing the melted polycondensate outside the extruder (second time segment) can include the melt filtration step to remove contaminant particles. A melt pump is preferably used to build up the necessary pressure. To this end, the melt pump and melt filter must be integrated into the process in such a way as to maintain the short retention time according to the invention.

The second time segment also involves the steps of molding and subsequently cooling to a temperature at which the reverse polycondensation reaction is essentially stopped.

The molding step here typically takes place in a die, through which the polycondensate melt is extruded. Die outlets of varying shape can here be used. Simple shapes, such as round, annular or polygonal, in particular essentially rectangular, shapes are possible, as are complex shapes. The essentially rectangular die outlets yield a band-shaped molded part owing to a high width to height ratio (e.g., W:H>=10). An essentially rectangular profile comes about at a low width to height ratio (e.g., W:H<10). A low width to height ratio is normally also present for the band-shaped, complex die outlets, giving rise to a profile. Such complex profiles have at least one partial surface arranged at an angle to another partial surface.

The size of the molded part as measured in width and height results from the dimensions of the die outlet and possible re-molding steps, for example, stretching or constriction. To achieve the desired shape of the molded part, it is generally known to adapt the shape of the die outlet to the properties of the polycondensate in such a way as to offset any dimensional changes after the die outlet, e.g., caused by strand expansion.

At least the width or height of the die outlet, and in particular of the molded part, normally exceeds 10 mm, especially 25 mm. In molded parts with a large width to height ratio, widths exceeding 100 mm, in particular 250 mm, are preferred. On the other hand, it is advantageous for at least one expansion of the molded part not to exceed 100 mm, in particular 50 mm, since the effect of cooling would vary too greatly between the outer wall and center of the molded part.

The length of the molded part depends on how often the molded part is cut essentially transverse to the manufacturing direction. In this case, the length can exceed the width or height by several orders of magnitude, as with films or bands, for example. In profile rods, the length still clearly exceeds the length or width. By contrast, the length of the molded part can be less than or at least comparable to its width or height with respect to panes or blocks.

The second time segment ends when all polycondensate has dropped below the melting point of the polycondensate T_(m). Rapid cooling is to take place for this purpose.

The initial cooling from the melting point of the polycondensate upon exiting the die (T_(D)) to a cooling point (T_(a)) is to take place at least at a cooling rate exceeding 300° C./min, preferably exceeding 600° C./min, especially preferably exceeding 1200° C./min, wherein t_(a)=0.9*T_(m). Cooling can take place via a cooling medium, for example water or liquid nitrogen, or through contact with a cold surface, e.g., a cooling roll or a cooling jacket around the profile. When using a cooling device structurally connected with the die, the transition from the die area into the cooling area is regarded as the point where the temperature of the device drops below the melting point of the polycondensate.

An initially higher cooling rate is very important primarily in thicker molded parts, not just to keep down the additional retention time outside the die, but also because degradation takes place at varying rates inside and outside the molded part. As the process continues, the molded part can then be cooled more gently to avoid stresses, or even specifically annealed, wherein temperatures in excess of the cooling point T_(a) are to be avoided for an extended time. The molded part can subsequently be rolled, formed, stamped out or cut into sections of defined length and/or width.

One feature of this invention is that the method according to the invention makes it possible to manufacture molded parts out of polyethylene terephthalate with a very low hydrolytic degradation and very slight quantities of acetaldehyde. For example, molded parts can be fabricated with less than 3 ppm, in particular less than 2 ppm, of acetaldehyde, without having to subject the molded part to a conditioning step to remove acetaldehyde.

Additional advantages, features and possible applications of the invention can be derived based on the drawing from the following description of examples, which are not to be regarded as limiting. Shown on:

FIG. 1 is the retention time of PET in a melted state permissible for an IV degradation from 0.81 to 0.76 as a function of the residual water content of a PET melt;

FIG. 2 a-2 e are examples for possible simple molded part or profile geometries, and on

FIG. 3 a-3 d are examples for possible complex molded part or profile geometries.

FIG. 1 shows the retention time of PET in a melted state permissible for an IV degradation from 0.81 to 0.76 as a function of the residual water content of a PET melt. The permissible retention times at a melting point of 285° C. are here specified for partial preliminary drying, limiting the IV degradation to the value of 0.05 acceptable for many applications. By contrast, preliminary drying to less than 100 ppm must be the goal in prior art, as mentioned at the outset.

FIG. 2 a-2 e show examples for possible simple molded part or profile geometries.

FIG. 2 a shows a molded part or profile with rectangular cross section, wherein the W:H ratio is large, e.g., >10. This is known as a band profile.

FIG. 2 b shows a molded part or profile with circular cross section. This is known as a round profile.

FIG. 2 c shows a molded part or profile with annular cross section. This is known as a band profile.

FIG. 2 d shows a molded part or profile with rectangular cross section, wherein the W:H ratio is small, e.g., about 1:1 to 3:1. This is known as a rod profile.

FIG. 2 e shows a molded part or profile with rectangular cross section and with beveled edges. This is known as a strip profile.

FIG. 3 a-3 d show examples for possible complex molded part or profile geometries, wherein FIG. 3 a is a cross profile, FIG. 3 b a dovetail profile, FIG. 3 c an angular profile and FIG. 3 d a U profile.

The method according to the invention makes it possible to also manufacture films or bands instead of molded pats or profiles. The films here preferably have a density of about 200 μm, and are in particular used as flexible packing films. The bands manufactured according to the invention preferably have a thickness of more than 200 μm, but can also be over 800 μm thick. The maximum thickness of these bands lies at roughly 5 mm. They are used in particular as rigid or semi-rigid packings.

EXAMPLE 1

A polyethylene terephthalate granulate manufactured with an antimony catalyst, an initially intrinsic viscosity of 0.84 dl/g and a moisture content of 600 ppm was melted and degassed in a 30 mm ring extruder, and subsequently removed through a profile die by means of a gear pump. The profile was cooled directly after the die outlet. Panes with a diameter of approx. 40 mm and a thickness of approx. 5 mm were fabricated from the profile. The throughput measured 200 kg/h. The retention time in the melted state in the extruder measured approx. 4 seconds, and the retention time in the melted state after the extruder measured approx. 12 seconds.

The manufactured panes had an intrinsic viscosity ranging from 0.79 to 0.80 dl/g, and an acetaldehyde content ranging from 1.5 to 1.8 ppm.

EXAMPLE 2

A polyethylene terephthalate granulate manufactured with a germanium catalyst, an initially intrinsic viscosity of 0.75 dl/g and a moisture content of 2000 ppm was melted and degassed in a 30 mm ring extruder, and subsequently removed through a profile die by means of a gear pump. The profile was cooled directly after the die outlet. Panes with a diameter of approx. 40 mm and a thickness of approx. 5 mm were fabricated from the profile. The throughput measured 200 kg/h. The retention time in the melted state in the extruder measured approx. 4 seconds, and the retention time in the melted state after the extruder measured approx. 12 seconds.

The manufactured panes had an intrinsic viscosity ranging from 0.71 to 0.72 dl/g, and an acetaldehyde content ranging from 2.0 to 2.5 ppm.

EXAMPLE 3

A polyethylene terephthalate granulate was processed as in Example 2. The throughput measured 120 kg/h. The retention time in the melted state in the extruder measured approx. 6 seconds, and the retention time in the melted state after the extruder measured approx. 18 seconds. The manufactured panes had an intrinsic viscosity in the 0.72 dl/g range.

EXAMPLE 4

A polyethylene terephthalate granulate was processed as in Example 2. The throughput measured 250 kg/h. The retention time in the melted state in the extruder measured approx. 3 seconds, and the retention time in the melted state after the extruder measured approx. 10 seconds. The manufactured panes had an intrinsic viscosity in the 0.71 dl/g range. 

1. A method for manufacturing a molded part, in particular a profile, out of a polycondensate, wherein the polycondensate is melted and later solidified again during the process, characterized in that the overall time for which the temperature of the polycondensate exceeds the melting point of the polycondensate during the process measures less than about 60 seconds.
 2. The method according to claim 1, characterized in that the overall time for which the temperature of the polycondensate exceeds the melting point of the polycondensate during the process measures less than about 30 seconds.
 3. The method according to claim 1 or 2, characterized in that the residual water content in the melt exceeds 200 ppm.
 4. The method according to one of claims 1 to 3, characterized in that the polycondensate is a polyester, in particular polyethylene terephthalate, or one of its copolymers.
 5. The method according to one of the preceding claims, characterized in that the initial form of the polycondensate is present as a loose material, e.g., granulate, flocks or chips, and has an apparent density ranging from 200 kg/m³ to 950 kg/m³.
 6. The method according to one of claims 4 to 5, characterized in that the polyethylene terephthalate is present as bottle recyclate.
 7. The method according to one of the preceding claims, characterized in that polycondensate starting material is subjected to partial preliminary drying before melting.
 8. The method according to one of the preceding claims, characterized in that it has a degassing step for removing volatile contaminants and/or decomposition products from the polycondensation melt.
 9. The method according to one of the preceding claims, characterized in that an extruder is used to melt the polycondensate.
 10. The method according to claim 9, characterized in that a two-screw or multi-screw extruder, in particular a ring extruder, is used to melt the polycondensate.
 11. The method according to one of claims 9 to 10, characterized in that the polycondensate is introduced into the extruder in a solid state, the polycondensate is heated to a temperature below the melting point, and the polycondensate is degassed and/or dried, characterized in that the polycondensate is degassed and/or dried in a solid state at a pressure of below atmospheric pressure and/or with the addition of an inert gas.
 12. The method according to one of claims 9 to 11, characterized in that the overall time for which the polycondensate is present as a melt during the process consists of a first time segment in which the polycondensate still remains in the extruder after melted in the extruder, and a second time segment for which the still melted polycondensate is processed outside the extruder,
 13. The method according to claim 12, characterized in that the first time segment measures less than about 15 seconds.
 14. The method according to claim 13, characterized in that the first time segment measures less than about 10 seconds.
 15. The method according to one of claims 9 to 14, characterized in that the processing of the melted polycondensate outside the extruder includes a melt filtration step
 16. The method according to one of claims 9 to 15, characterized in that the processing of the melted polycondensate outside the extruder involves the use of a melt pump.
 17. The method according to one of claims 9 to 16, characterized in that the processing of the melted polycondensate outside the extruder involves the use of a molding die.
 18. The method according to claim 17, characterized in that the molding die has an outlet with a size exceeding 10 mm, in particular exceeding 25 mm, in at least one direction (width or height).
 19. The method according to claim 17 or 18, characterized in that the polycondensate is cooled at an initial rate exceeding 300° C. per minute after exiting the molding die.
 20. The method according to claim 19, characterized in that the polycondensate is cooled at an initial rate exceeding 600° C. per minute, in particular exceeding 1200° C., after exiting the molding die.
 21. The method according to one of the preceding claims, characterized in that an additive is incorporated into the polycondensate.
 22. A molded part, in particular a profile, manufactured out of a polycondensate using a method according to one of the preceding claims.
 23. The molded part according to claim 22, characterized in that the average acetaldehyde content of the molded part measures less than 3 ppm, in particular less than 2 ppm. 